A quadrupole ion trap is a special device. It may serve as a device to store ions which confines gaseous ions within the region of the quadrupole field of the ion trap in a certain time period, and may also function as a mass analyzer for a mass spectrometer so as to conduct mass spectrometric analysis. In addition, such an ion trap possesses a broad mass range and a variable mass resolution. The quadrupole electrostatic field is generated via introducing a RF (radio frequency) voltage, a DC voltage or a combination signal thereof onto individual electrodes of the ion trap. Traditional ion traps consist of two types of electrode, that is, an annular electrode and an end cover electrode. A typical electrode shape is hyperbolic so as to generate a significant quadrupole field.
Ion traps in early days are three-dimensional ion traps, whose quadrupole field is generated along the r and z directions (in a polar coordinate system). In this quadrupole field, ions are acted upon by linear forces, such that those ions with a mass-to-charge ratio within a certain range are captured and stored in the ion trap. The most typical three-dimensional ion trap is composed of three hyperbolic electrodes, e.g., an annular electrode and two end cover electrodes. Such a device is commonly referred to as a Paul ion trap or a quadrupole ion trap. A columnar ion trap is a simpler ion trap which is composed of an annular electrode with its inner surface being columnar and two end cover electrodes in a flat plate structure.
Both a Paul ion trap and a columnar ion trap suffer most from that only a small number of ions are captured in the trap, and that the capture ratio of incident ions ionized outside the trap is extremely low. In order to suppress the space charge effect so as to attain a higher resolution, a commercial mass spectrometer only captures 500 ions or even less in a typical experiment. The ions that are introduced into the ion trap through the inlet on the end covers will be subjected to the RF field, and only those introduced at proper RF phase could be efficiently captured and stored in the trap. The capture ratio is less than 5% for continuously incident ions, and in most cases it is much lower than 5%.
To solve the above problem, another type of ion trap, that is, a linear ion trap is proposed. Such a linear ion trap is composed of a plurality of elongate electrodes that are in a parallel arrangement. The electrode system will determine the volume of the ion trap. A two-dimensional quadrupole field may be generated in the plane perpendicular to the central axis of the ion trap by applying RF voltage and DC voltage to the electrodes. Since a strong focusing of ions is just realized in a two dimension topology, the captured ions may be distributed around the central axis, and the number of ions that are captured is significantly increased. U.S. Pat. No. 5,420,425 describes a two-dimensional linear ion trap which is composed of three sets of quadrupole electrodes, in which the quadrupole set in the middle is main quadrupole electrode. One pair of main quadrupole electrodes thereof is provided with slots, through which ions may be introduced in and discharged out. The two sets of quadrupole electrodes on the both ends may function to axially restrict the motions of the ions captured in the trap, and also may improve the quadrupole field inside the main quadrupole electrodes. When individual electrodes are hyperbolic electrodes, an almost ideal quadrupole field may be attained.
All above mentioned ion traps, except the columnar ion trap, demand a precise machining process, such as manufacture and assembly, etc. Nevertheless, such high precision processes are very complicated, and therefore become a predominant factor that impairs the applicability of the small-size portable ion trap mass spectrometer.
U.S. Pat. No. 6,838,666 B2 proposes a linear rectangular ion trap, in which four rectangular flat plate electrodes are arranged in parallel to the axis so as to enclose an ion trap with a rectangular cross section. A RF voltage and a DC voltage are applied to the individual flat plate electrodes to generate a quadrupole field in the ion trap, such that ions are focused onto a two-dimensional plane. Axial restriction upon the motions of ions is realized by introducing end electrodes. The rectangular ion trap solves the problem of high precision mechanical processes of the linear ion traps, while at the same time it brings about a new issue, i.e., a substantial uncertainty in ion motions resulting from the fact that high order fields residing in the quadrupole field produced by the four flat plate electrodes, such as a dodecapole field and an icosapole field. In this way, the mass resolution of the ion trap mass spectrometer is impaired.
Early studies of field shape demonstrated that the introduction of higher order fields tended to impair the mass resolution of a quadrupole mass spectrometer. However, the latest researches show that the mass resolution of a quadrupole mass spectrometer may be improved effectively by properly introducing components of higher order fields. For example, in U.S. Pat. No. 6,897,438 B2, parameters of a quadrupole electrode system (such as the ratio of radii or fields of two pairs of electrodes) are changed to introduce an octopole field into a quadrupole filed, such that the mass resolution is improved. This patent only discloses a method to introduce an octopole field into a quadrupole field, that is, changing radii of the electrodes or radii of the fields, without mentioning any method for introducing other higher order fields.
In summary, a two-dimensional ion trap is a linear ion trap that can realize a large capacity and solve the problem that the number of ions captured by a three-dimensional ion trap is small and thus the capture efficiency is low. However, an existed two-dimensional ion trap either demands high precision machining, or contains significant higher order fields. These disadvantages may impair the development of small-size portable ion trap mass spectrometers. On the other hand, introduction of higher order fields should be taken into considerations in the studies of field shape optimization for quadrupole mass spectrometers. However, the prior patents only discuss the introduction of an octopole field and propose no practical technical solutions to introduce other higher order fields. Investigations for an ion trap and a mass spectrometer thereof having flexible structures, being easy to be manufactured, and conveniently attaining an optimized field shape will significantly promote the development of small-size portable ion trap mass spectrometers.
In a mass spectrograph is often employed a multipole electrode system of an ion optical system. In the field of mass spectrometry, a multipole electrode system is generally employed as an ion optical system. For example, quadrupole electrodes, hexapole electrodes, or octopole electrodes, etc., is applied as an ion lens or an ion guiding system. The field shape in the regions of such multipole electrodes are very important for ion transferring and focalizing.
Most electrodes in the prior art multipole electrode system are cylindrical or hyperbolic electrodes. It is well known that hyperbolic electrodes are difficult to be manufactured and assembled in a high accuracy. As for cylindrical electrodes, even though they may be manufactured in a high accuracy, they cannot be assembled in a high accuracy. In this sense, the manufacture and assembly limit its performance.
U.S. Pat. No. 6,441,370 B1 proposed a rectangular linear multipole electrodesystem, which may be used for ion guiding and may be used in ion traps. This multipole electrode system employs an electrode with a rectangular section. The surface of the rectangular electrode is superimposed with a surface layer, which functions to improve the field shape. The manufacture and assembly will be greatly simplified by employing a rectangular electrode. However, this patent did not disclose the concrete technical solution capable of improving the field shape. The surface layer can only improve the field shape qualitatively, and it can not realize this in a quantitative way.
If the desired multipole field shape could not be realized, the machining (including manufacture and assembly) of the multipole electrode system could not be performed in a high accuracy, then the performance of the multipole electrode system, and hence the ion optical system in a mass spectrograph will be seriously affected. Therefore, it is desirous to develop a multipole electrode system, which presents an optimized field shape and a flexible structure, is easy to be manufactured, and has a low manufacture cost, so as to construct such an ion optical system that has a stable performance and is capable of controlling the ion trajectories precisely.